In a conventional permanent magnet (PM) rotating machine having a rotor and stator, rotor magnets normally are mounted on the surface of the rotor back iron and produce an air gap flux density equal to the area of one of the permanent magnet's pole face area, as lowered by the air gap reluctance. Further, the magnets are located on the rotor in a manner where two permanent magnets face into three stator poles to accommodate a conventional three-phase lap wound motor/alternator or generator design. Since the flux from the permanent magnets in this conventional design is shared by three poles, the effect is diminished with regards to increasing the gap flux density. Further, if one pole in such a convention design is shorted, all three poles are shorted, which will cause the machine to lock. This does not provide for a fault tolerant design, and any imbalance on one of the phases will distort the other phases.
With the rising cost of rare earth permanent magnet materials, rotating machine designers are looking for solutions that will reduce the amount of rare earth material used without sacrificing power density. A conventional way of achieving this goal is to increase the number of stator teeth that produce torque over the 360 degrees [2 pi radians] they occupy.
One such machine topology is a single phase permanent magnet synchronous motor. A drawback with a single phase permanent magnet [PM] synchronous motor/generator is that all of the rotor and stator teeth come into and out of alignment at the same time or angular intervals, producing their minimum and maximum torque (motor) or power (generator) values at the same time. Therefore, the average power (mechanical power/torque or electrical power) is lower than the desired optimal torque or power.
A multiphase permanent magnet machine will produce a higher average torque or power since each phase will contribute to the torque or power at different angular intervals. However, a distributed or lap wound permanent magnet multiphase machine will have one or more stator teeth that do not produce torque or power over an angular interval since more than one tooth forms a stator pole. This lowers the number of torque or power producing teeth over 2 pi radians. In addition, the phases in a permanent magnet distributed or lap wound multiphase machine share the same flux sources (i.e. the permanent magnets), which limits the amount of available energy stored in the magnetic field for a given phase since this energy is shared by all of the machine's phases. A concentrated wound multiphase machine increases the number of teeth producing torque or power but does not address the problem with the phases sharing the same flux source (i.e. the permanent magnets).